The InAlN/GaN heterostructure has been predicted to be capable of delivering high current and high power densities due to its strong spontaneous polarization. An InxAl1-xN barrier (where x is between 0 and 1), moreover, can be grown lattice matched to a GaN buffer layer, such as at an indium mole-fraction x=0.17 (about a 17% indium molar fraction and 83% aluminum fraction of the InAlN), allowing fewer structural defects than the AlGaN/GaN lattice mismatched heterostructure, providing less strain and therefore decreased current collapse (or slump) following multiple on/off switching events or cycles. Although there has been significant progress in the development of InAlN/GaN high electron mobility transistors (“HEMTs”, also referred to as heterostructure field-effect transistors (“HFETs”)) for high drive current, low on-resistance, and reliability in ultrahigh temperature environment, the comparatively low breakdown voltage (“BV”) of these InAlN/GaN devices severely limits the device performance and their potential applications.
As a result, one of the most significant problems with InAlN/GaN devices is the comparatively very low breakdown voltages, or early breakdown, which is typically less than 200V. This is not suitable for many applications, such as the power switches in smart grids, or hybrid or electric cars, factory automation, wind turbines and wind mills, for example.
Breakdown voltages ranging from 350 to 650 V and relatively high leakage currents ranging from 10−5 to 10−4 A/mm have recently been demonstrated in normally-on devices. BV and leakage current of normally-off InAlN/GaN devices, however, are even worse than their normally-on counterparts. For example, one of the very few normally-off InAlN/GaN MISHFETs showed a BV of 345 V at LGD=4.5 μm with a comparatively high leakage current of 0.8×10−3 A/mm. Normally-off operation, however, is much preferred for simpler circuits as well as for safety concerns, especially for high voltage power conversion applications.
Accordingly, a need remains to significantly improve the breakdown voltage and reduce the leakage current of normally-off InAlN/GaN devices. In addition, such a normally-off InAlN/GaN device should still provide suitable performance characteristics for other device parameters, such as sufficient frequency response, a comparatively high on-current, and an absence of current collapse when repeatedly switched on and off.